The present invention relates to image registration and fusion and is particularly related to a method and apparatus using a non-rigid technique for registration and fusion of images. More specifically, the image registration and fusion is adapted to compensate for physiological motion during imaging. The present invention finds particular application in conjunction with diagnostic medical imaging and will be described with particular respect thereto.
In the practice of medicine, various techniques or imaging modalities are available for obtaining diagnostic images of the human body. Each of the imaging modalities may employ different methods and apparatus for acquiring data from an imaging subject and processing the acquired data into suitable images. The various imaging modalities yield images having features that are characteristic to the specific imaging technique.
Since the different imaging modalities have characteristic features related to their particular data acquisition and image processing methods, a particular modality may be more useful for obtaining specific types of diagnostic information. For example, functional imaging modalities include scintigraphy, functional MRI (fMRI) and nuclear medicine imaging techniques such as SPECT and PET. In addition, some lesser used functional techniques include perfusion MRI (pMRI), functional CT (fCT), electro impedance tomography (EIT) and magnetic resonance elastography (MRE). These functional modalities can provide imaging information showing primarily metabolic or functional information and some structural features of the imaged subject matter. However, images generated using some of these modalities is generally directed to a specific region, physiological system or organ of interest and yields little information about specific anatomical structures surrounding the subject matter of interest. For example, in nuclear medicine imaging techniques, a radiopharmaceutical is injected into a patient. Specific radiopharmaceuticals are selected to provide images for particular diagnostic imaging tests. Some radiopharmaceuticals concentrate in a particular region of interest, such as the circulatory system, the heart, brain or other organs and causes radiation to be emitted from the region of interest. The radiation emitted from within the patient is detected by the imaging equipment and is used to generate the diagnostic images. The images resulting from nuclear imaging techniques provide substantial information about the region of interest but generally do not show skeletal structures or other nearby organs such as the lungs when the radiopharmaceutical is selected to emphasize the heart. A physician may also require image information about the structure nearby the particular region of interest of the patient to make a more accurate diagnosis or administer a selected treatment.
When a physician requires images of anatomical structure, other medical imaging modalities can be used. For example, anatomical modalities include X-Ray, CT, MRI, ultrasound, portal images and video sequences obtained by various scopes such as laparoscopy or laryngoscopy. Some derivative techniques include magnetic resonance angiography (MRA), digital subtraction angiography (DSA) and computed tomography angiography (CTA). Images obtained from these modalities can be used to provide suitable images for general anatomical structure within an examination region.
When images from more than one imaging modality are available, it is often desirable to combine the information in the separate images from the different modalities into a single image. In addition to multimodality registration and fusion, it is sometimes valuable to combine images from a single modality. Monomodality registration can be useful for treatment verification by comparison of pre and post intervention images, comparison of ictal and inter-ictal (during and between seizures) SPECT images, growth monitoring using time series of MR scans on tumors or X-ray time series on specific bones as well as the area of patient staging, where the patient contour, organ positions and sizes could be different due to time, changes in body habitus, and different acquisition positions and or protocols.
Rather than side by side comparison, the multimodality or monomodality images may be superimposed upon one another to correlate the location of specific image features relative to one another. Superposition of images of specifically related subject matter involves registration of the images and fusion of the images. Registration generally involves spatial alignment of the images and fusion is performed to produce the integrated display of the combined images. The combined or fused images might be, stored, displayed on a computer screen or viewed on some form of hard output, such as paper, x-ray film, or other similar mediums.
Various methods are known for registering images from different imaging modalities. However, registering images with both ease and accuracy is a problem associated with these methods. For example, images can be registered manually by an operator or medical professional. However, this method is generally not very accurate since there is oftentimes insufficient common information between the images to use as reference points.
Another registration method involves the use of markers (fiducials) or stereotactic frames. When using these extrinsic methods, markers or reference frames are placed next to or onto a patient during imaging. The patient is imaged in one modality then transported to the other modality for imaging. The markers or frames are visible in the images to be combined. Precisely fixing the location of the markers relative to the patient's body can be problematic. The patient may move slightly between scans and during scans, and if there is patient movement relative to the markers, it becomes difficult to accurately register the resulting images.
Intrinsic methods rely on patient generated image content. Some examples of these registration methods includes identification of salient points or landmarks, alignment of segmented binary structures such as object surfaces and utilizing measures computed from the image grey values (voxel based).
One of the challenges in image fusion, regardless of the presently available method selected, is that the images may never align well using rigid body registration methods due to physiological movements such as diaphragm motion. This is particularly true when the scan time to acquire image data for a subject is different. For example, a suitable image dataset may be obtained in a single breath hold for a CT image while a PET scan may require many respiratory cycles throughout the data collection period. The physiological motion during the longer nuclear scan can make it difficult to register and fuse the PET and CT images. This motion causes inaccurate registration and fusion of the images.